1. Field of the Invention
This invention relates to apparatus for measuring electromagnetic signal characteristics and predicting a system response to a change in signal input; and, more particularly, to an adaptively controlled electromagnetic signal analyzer utilizing a biased semiconductor junction.
2. Description of the Related Art
Accurate measurement and control of electromagnetic signals is important in signal processing applications. Current approaches for achieving high measurement accuracy include: (1) using costly precision hardware and (2) calibrating the measurement errors of more moderately priced hardware. Although error calibration is generally more cost effective than using precision hardware, calibration accuracy associated with moderately priced components is limited.
Instantaneous frequency detectors are presently available in the electromagnetic signal processing market. Currently, biased semiconductor junctions are used to perform attenuation and detection separately in signal processing.
For measurement of the relative amplitude and phase between signals, the test instrument market is now dominated by heterodyne network analyzers with computerized calibration. In the past decade, much work has been completed on 4-port and 6-port network analyzers because of their potential for replacing the costly and complex electromagnetic and analog circuitry of a heterodyne analyzer by simple power detectors. Problems in achieving accurate calibration of power detectors over a wide dynamic range have limited the success of 4-port and 6-port network analyzers in the test instrument market.
For measurement of electromagnetic signal amplitude versus frequency, the test instrument market is dominated by dedicated heterodyne spectrum analyzers. Dual-purpose scalar network analyzers (SNA) which also perform scalar spectrum analyzer measurements are available. Dual use of circuitry common to network and spectrum measurements offers cost savings compared to separate spectrum and network analyzers. Fast Fourier Transform (FFT) processors also are available to calculate the absolute amplitude and phase versus frequency of a signal, but the frequency range is limited by available analog-to-digital converter (ADC) speeds.
Complex phasor modulators are available for controlling the magnitude and phase of an electromagnetic signal. Complex phasor modulators of the prior art are limited in the level of precision signal control over frequency and temperature, and they introduce signal distortion. This makes the prior art unsuitable for precise interference cancellation of amplitude-modulated or frequency-hopping signals.
Precise adaptive interference cancellation is needed in communication systems which must operate a radio transmitter in close proximity to a radio receiver. In this interference reduction approach, a sample signal is coupled from the interfering transmitter, passed through a controlled reference path, and then summed with the signals at the receiving antenna. The signal at the receiver is measured and the amplitude and phase of the interference in the reference path is adjusted to cancel the interference in the receiver.
For interference cancellation in a system with a narrow-band signal centered about a hopping carrier frequency, the control values at one carrier frequency may require adjustment at another carrier frequency. Fast frequency-hopping systems require the speed of a lookup table for determining control adjustment between hops. However, system changes or hostile jamming may degrade table accuracy; and there may be little, if any, time available for table update. Also, conventional methods for updating the table are based on calculating the correlation of signals by integrating over time the product of the signals. However, the accuracy of this signal correlation calculation breaks down for a fast hopping system. That is, the time at each frequency is too short to accurately define correlation between signals.
In summary, the accurate modeling and calibration of moderately priced electromagnetic signal amplitude, phase, and frequency measurement hardware in the prior art is limited. Also, complex phasor modulators of the prior art introduce unacceptable signal distortion in the control of amplitude-modulated and frequency-hopping electromagnetic signals. Finally, interference cancelers of the prior art based on long term signal correlation at a single carrier frequency are unsuitable for fast frequency-hopping systems.